Demand for an optical communication transmission apparatus is increasing in a context of increase of the communication traffic in recent years. Installation of an optical communication transmission apparatus not only in an optical relay node incorporated in a trunk network but also in a recent local network is advanced actively. Further, an optical network is formed also in a subscriber system. In this manner, the optical communication system has a significant role in the information network in the world. In an optical network, an optical amplification relay is used dominantly which includes a wavelength multiplexing optical amplifier such as an EDFA (Erbium Doped Fiber Amplifier) provided for each of transmission lines so that it implements increase of the capacity and long-distance transmission at a low cost and in high reliability.
In an optical amplification relay system, under the condition that the relay loss is high from such a cause that the transmission line is long, the input level of signal light to an optical amplifier is low. Therefore, there is the possibility that the signal to noise ratio (SN ratio) may degrade, resulting in degradation of the transmission characteristic. As a method for avoiding this, it is effective to apply transmission line distributed Raman amplification which utilizes an amplification action using an effect of induced Raman scattering when pumping light is injected into a transmission line.
A distributed Raman optical amplifier (Distributed Raman Amplification; DRA) has already been put to practical use as an effective implementation method for achieving transmission line distributed Raman amplification. By applying the distribution Raman optical amplifier, the input level to an optical amplifier such as an EDFA or the like increases to increase the SN ratio thereby to improve the transmission characteristic and increase the number of spans which can be transmitted. Further, in the Raman amplification, not optical output fixing control (Automatic Level Control; ALC) but gain fixing control which does not require transmission of wavelength information between nodes must be carried out in order to be ready for dynamic fluctuation of the number of wavelengths.
The gain of Raman amplification can be controlled by increase or decrease of the pumping power. A conventional technique relating to gain fixing control of a DRA is disclosed in Japanese Patent Laid-Open No. 2004-193640 (hereinafter referred to as Patent Document 1). In particular, in Patent Document 1, a technique is disclosed wherein transmission line distributed Raman amplification is carried out by gain fixing control irrespective of such conditions as the gain efficiency of an optical fiber which forms a transmission line and the loss of a connection path or of the signal light level so that the output wavelength characteristic of the Raman amplification is kept fixed. In the technique disclosed in Patent Document 1, it is described to determine a Raman gain utilizing reference light which does not undergo Raman amplification and control pumping light so that the determined Raman gain becomes equal to a predetermined value.
It is to be noted that also techniques disclosed in Patent Documents 2 to 4 given below relate to the present invention.    (Patent Document 2) Japanese Patent Laid-Open No. 2004-80301    (Patent Document 3) pamphlet of International Publication No. 2002/021204    (Patent Document 4) Japanese Patent Laid-Open No. 2002-252595
However, while it is necessary to monitor the optical main signal level inputted through an optical transmission line using a monitor circuit formed from a light reception device and so forth in order to determine the Raman gain, the signal of a result of the monitoring includes not only circuit noise by a monitor circuit section but also noise by the Raman amplification. If such a monitoring result signal which includes noise as just described is used for the gain derivation described above, then this makes a cause of an error in a derived gain value and also may make an obstacle to enhancement of the accuracy of gain control.